Dan O'Brien stars in the NBC multi-camera comedy "Whitney" as Mark, a police officer who lives next door to Whitney (Whitney Cummings) and Alex (Chris D'Elia). He claims to be the ultimate player and eternal bachelor, but likes to talk a big game.

O'Brien has worked extensively in theater in New York City with The Atlantic Theater Company, MCC, The Present Company, The Flea Theater and The Vineyard Theater, among others. In the UK, O'Brien has performed at the Camden People's Theater in London and the Edinburgh Fringe Festival, garnering a Scotsman Fringe First Award for his work. With his wife and playwright-in-residence, Julie Shavers, he has directed and produced four plays, most recently "Silver Bullet Trailer," which was developed with the Royal Court Theater and Present Company and performed at The Ohio Theater in New York.

O'Brien's film credits include Rebecca Gilman's "Spinning into Butter" and the upcoming David Letterman-produced "The Greatest Movie Ever Made." Recent television appearances include "How I Met Your Mother," "Romantically Challenged" and "Delocated."

O'Brien is a graduate of the University of Notre Dame. He currently resides in Los Angeles with his wife and children.